Civil Engineering July 2021 | Vol 29 No 6

30 July 2021 Civil Engineering at Ganta and Gbarnga. The average AADT over the monitoring period is 2 538 (at Gbarnga) with 18.3% being heavy vehicles. Axle loading Axle loading was considered in the pave- ment design by converting the loading of each vehicle type to equivalent standard axle loads (ESALs) following the principles in AASHTO, 1993. The equivalent load factors (ELF) given in the contract were re-assessed by considering the allowed legal axle loads as stipulated in the Republic of Liberia’s Heavy Duty Axle Load Regulations (2016). A summary is given in Table 1. In view of significant overloading that often occurs where there is limited law enforcement, as well as the absence of recent axle load weighing data, it was recommended that increased loading fac- tors be adopted for the overlay design. Traffic growth rate In terms of the contract documents, the pavement design for the rehabilitation is based on the following predicted annual growth rates: Q Q 2009 to 2012: 35% per year Q Q 2012 to 2014: 10% per year Q Q 2014 to the end of the 20-year design life: 5% per year The analyses of the actual traffic counts indicate average annual growth rates of 23.9% and -7.1% for Gbarnga and Ganta respectively, between 2009 and August 2020. It was evident from the analyses that the actual growth in traffic has been erratic over the last 11 years and is not following the predicted values. In view of these uncertainties, and given the developing economy of Liberia, it was rec- ommended that an annual traffic growth exceeding the proposed 5% per year be adopted for designing of the periodic maintenance intervention. Design traffic The revised cumulative design traffic was calculated based on the actual traffic counts carried out from 2009 to 2020. Three scenarios were considered as part of a sensitivity analysis: Q Q Scenario 1: using the ELFs and percentage of annual traffic growth as per the contract documents i.e. 5% per year from 2020 to 2034 Figure 10 Cumulative ESALs over the pavement design life (Scenario 3) Cumulative ESALs 3.00E+07 2.50E+07 2.00E+07 1.50E+07 1.00E+07 5.00E+06 0.00E+00 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 Year 2.75E+07 2.54E+07 2.34E+07 2.14E+07 1.96+07 1.78E+07 1.62E+07 1.46E+07 1.31E+07 1.16E+07 1.03E+07 8.94E+06 7.70E+06 6.51E+06 5.38E+06 4.30E+06 3.27E+06 2.30E+06 1.37E+06 2.56E+07 2.34E+07 2.14E+07 1.94E+07 1.75E+07 1.57E+07 1.40E+07 1.24E+07 1.08E+07 9.36E+06 7.96E+06 6.62E+06 5.35E+06 5.66E+06 7.73E+06 4.76E+06 7.13E+06 Gbarnga – forecast Ganta – forecast Ganta – actual Gbarnga – actual Gbarnga – predicted 5% Ganta – predicted 5%